1. Field of the Invention
This invention relates generally to a modulation system for a wireless transmitter more particularly, the invention relates to a direct modulator for a wireless transmitter.
2. Related Art
With the increasing availability of efficient, low cost electronic modules, mobile communication systems are becoming more and more widespread. For example, there are many variations of communication schemes in which various frequencies, transmission schemes, modulation techniques and communication protocols are used to provide two-way voice and data communications in a handheld, telephone-like communication handset, also referred to as a portable transceiver. The different modulation and transmission schemes each have advantages and disadvantages.
As these mobile communication systems have been developed and deployed, many different standards have evolved, to which these systems must conform. For example, in the United States, many portable communications systems comply with the IS-136 standard, which requires the use of a particular modulation scheme and access format. In the case of IS-136, the modulation scheme is narrow band offset π/4 differential quadrature phase shift-keying (π/4-DQPSK), and the access format is TDMA.
In Europe, the global system for mobile communications (GSM) standard requires the use of the gaussian minimum shift-keying (GMSK) modulation scheme in a narrow band TDMA access environment, which uses a constant envelope modulation methodology. Other modulation formats use minimum shift keying (MSK), frequency shift-keying (FSK) and other shift-keying modulation methodologies.
Furthermore, in a typical GSM mobile communication system using narrow band TDMA technology, a GMSK modulation scheme supplies a low noise phase modulated (PM) transmit signal to a non-linear power amplifier directly from an oscillator. In such an arrangement, a highly efficient, non-linear power amplifier can be used thus allowing efficient modulation of the phase-modulated signal and minimizing power consumption. Because the modulated signal is supplied directly from an oscillator, the need for filtering, either before or after the power amplifier, is minimized. Further, the output in a GSM transceiver is a constant envelope (i.e., a non time-varying amplitude) modulation signal.
The current GSM mobile communication system employs a modulation format that converts the transmit signal to an intermediate frequency (IF) before upconversion to radio frequency (RF). In such a system, one or more mixers and frequency sources (oscillators), are used to perform the upconversion. Unfortunately, these components are costly, consume significant power, consume a significant amount of area on the device, and typically require an “off-chip” filter to tune the mixer.
Furthermore, regardless of the type of modulation methodology employed, as virtually all of these portable communication devices operate using a rechargeable power source, such as a battery, it is desirable to minimize the amount of power consumed by the portable communication device so that the operating time of the portable communication device may be maximized.
In the portable transceiver, the oscillator that is used to develop a signal at a particular frequency that is used to convert the transmit signal from baseband to the proper transmit frequency, and to convert the frequency of a received signal to a baseband signal, consumes a significant amount of power. The oscillator generates what is referred to as a “local oscillator” signal, or “LO” signal. Such an oscillator may be what is referred to as a “voltage controlled oscillator,” or “VCO,” a current controlled oscillator (CCO), or another type of oscillator. A VCO develops a desired frequency based upon a voltage signal supplied to a bias port of the VCO. Further, the LO path typically includes at least one voltage gain amplifier.
In systems that use an IF transmit signal, the VCO is used to provide the local oscillator and another VCO is used to upconvert the transmit signal to RF. Further, a mixer is typically used to generate the in-phase (I) and quadrature (Q) components of the transmit signal. Unfortunately, the mixer, the IF VCO, and the voltage gain amplifier consume significant power.
In systems that use a direct upconversion methodology, many of the components are implemented using passive analog elements, which are susceptible to pole/zero location mismatch and impedance mismatch caused by manufacturing process and temperature variations. These analog elements are typically located in the phase locked loop (PLL) that is part of the upconverter.
Therefore it would be desirable to reduce the power consumption of the modulator, while allowing direct upconversion of the transmit signal from baseband to RF, while also minimizing pole/zero location mismatch between passive loop filter components in the PLL and digital data pre-emphasis data.